In recent years, optical information recording media have been required to have larger and larger information recording capacities for processing of a vast amount of information such as images. Solutions to this are a method that involves the use of a super-resolution technology, which is a technology for improving information processing during reproduction, and a method that involves the use of a multilayer optical information recording medium having multiple information recording layers on each of which recording/reproduction can be performed.
The super-resolution technology is a technology for reproducing a signal of a mark length (determined according to a laser wavelength and the numerical aperture of an optical system) that is less than or equal to an optical resolution limit of a reproducing device. Since this makes it possible to perform recording with a smaller mark length, there is an increase in substantive recording density. This is attributed to the fact that it is not a recording technology but a reproduction technology that matters in terms of achieving a higher density.
Of these technologies, the super-resolution technology is described first.
Conventionally, there have been proposed a large number of optical information recording media (hereinafter referred to as “super-resolution media”) for reproducing a signal of a mark length that is shorter than an optical system resolution limit of a reproducing device.
A known example of such a technology cannot be utilized for the reproduction of non-rewritable information recorded on a read-only medium with depressions in and protrusions on a substrate, but is compatible with a rewritable optical magnetic recording medium having a recording film made of a magnetic material and is used in reproducing information recorded on the recording film along a magnetization direction (see Patent Literature 1).
Another known example of such a technology is compatible with a read-only medium as well as a rewritable optical recording medium, and provides, on a reproduction light incidence surface of a reflecting film, a mask layer constituted by a thermochromic pigment layer that changes in optical characteristic (transmittance) according to temperature (see Patent Literature 2).
As will be described later, the mask layer is a layer that causes a super-resolution phenomenon, for example, by pseudo-narrowing a laser spot.
Each of these optical information recording media utilizes a temperature distribution attributed to a light intensity distribution in a laser spot formed by a reproduction laser striking a reproduction surface of the optical information recording medium.
More specifically, such an optical magnetic recording medium as that disclosed in Patent Literature 1 has a reproduction layer provided on a recording layer. Moreover, during reproduction, a magnetic field of the recording layer is transferred onto the reproduction layer only in a high-temperature portion within a laser spot. This makes it possible as a result to reproduce a signal of a mark length that is shorter than the optical resolution limit.
Further, in such an optical recording medium as that disclosed in Patent Literature 2, there occurs a temperature or light intensity distribution within a reproduction laser spot on the reproduction layer that is close to the reproduction light incident surface than the reflecting layer, whereby there occurs a distribution of optical characteristics within the laser spot.
For example, in a case where the reproduction layer is made of a material that becomes higher in transmittance as temperature rises, the reproduction layer becomes higher in transmittance in a high-temperature portion, so that a laser spot occurring on the reflecting layer is pseudo-narrowed. This makes it possible as a result to reproduce a signal of a mark length that is shorter than the optical resolution limit.
However, since the super-resolution reproduction technology pseudo-narrows a laser spot, there is a decrease in efficiency in the use of reproduction light (there is of course a decrease in reproduction light). This imposes a limit on the narrowing of a laser spot, and an improvement in recording density is at most approximately twice as high in terms of liner density.
Next, a multilayer optical information recording medium is described.
As disclosed in Patent Literature 3, for example, a multilayer optical information recording medium is structured to have (i) information recording layers such as a first information recording layer and a second information recording layer provided in this order from a reproduction light incident surface and (ii) an intermediate layer, made mainly of resin, which separates the information layers from each other.
In such a structure, an information recording layer other than the information recording layer located furthest from the reproduction light incident surface is a half-transparent layer that transmits reproduction light. This allows reproduction light incident on the reproduction light incident surface to be focused on each information recording layer. Therefore, this multilayer optical information recording medium can be said to be an optical information recording medium whose information recording density can increase as the total number of information recording layers increases.
It should be noted that the most common optical information recording medium that is used for this technology is a single-sided reproduction two-layer DVD-ROM.
However, an increase in the number of layers of a multilayer optical information recording medium makes it difficult to produce the multilayer optical information recording medium, thus making the medium very expensive. The following explains a reason for the difficulty in production with reference to an example of a method for producing a multilayer optical recording medium.
In the production of a multilayer optical information recording medium, e.g., a DVD, which is currently most common, it is only necessary to form a first information recording layer such as a recording film and a reflecting film on a substrate in vacuum, place back the resulting substrate into the atmosphere, and then join on top of the substrate a substrate on which a second information recording layer has been formed in a similar manner, as long as the number of layers is up to two.
However, in a case where the number of layers increases to three, e.g., in a case where the number of layers is three, the first information recording layer formed in vacuum is spin-coated in the atmosphere with an ultraviolet curing resin or the like that is to serve as an intermediate layer. Next, by joining a plastic stamper on top of the ultraviolet curing resin in the atmosphere, curing the ultraviolet curing resin by irradiating it with ultraviolet rays, and then removing the stamper, (i) grooves for tracking and (ii) depressions and protrusions such as pre-pits on the basis of whose sequence information has been recorded are transferred onto a surface of the intermediate layer (called “2P method”).
Then, again in vacuum, a second information recording film needs to be formed on the intermediate layer onto which the depressions and protrusions such as the pre-pits have been transferred; furthermore, in the atmosphere, a substrate on which a third information recording layer has been formed needs to be prepared in a similar manner to the substrate provided with the first information recording layer, and be joined on top of the second information recording layer.
In this way, the medium is produced through very complicated steps that require the medium to move between vacuum and the atmosphere.
Further, since each layer has a different film structure for adjusting its reflectance and, during normal mass production, is formed on each medium traveling in one direction through the production line, as many vacuum film-forming apparatuses as information recording layers are required. Moreover, such film-forming apparatuses are very expensive, and are high in running cost among apparatuses that are used in the production of optical information recoding media. Therefore, it has been substantially difficult in terms of cost to increase the number of information recording layers to three or larger.
Despite the above two methods thus proposed as main methods for increasing the density of an optical information recording medium, there have been problems with each of these methods as mentioned above. For this reason, a multilayer super-resolution technology has come to be proposed as a technology which has the advantages of both methods and which can effectively increase the information recording density of an optical information recording medium (Patent Literature 4).